The present invention relates to a method for monitoring a drive device for a standstill condition.
There are machine parts (for example an inclined bed in the case of a rotary machine or a lift cage in the case of normal conveying equipment) which are driven by means of conventional electrical drive devices and in which the state “suspended load” can occur. That is to say there is a state in which a minimum torque needs to be applied by the drive in order to counteract the force of gravity, which pulls the load downwards.
In the event of failure of or another fault in the electrical drive device, said electrical drive device cannot apply the required torque. If no further measures were to be taken, the lift cage could fall down, for example, and people and objects may be endangered.
In order to provide protection against such risks, safety devices are known, for example releasable brakes, which engage in the event of a fault and in the event of failure of the power supply and transfer the suspended load into a safe state. In order to detect such faults, when at a standstill multi-channel, redundant safety systems and components are used which cause one or more safety devices to be triggered.
The transfer into the safe state can naturally only be brought about when a fault which has occurred is also detected. Depending on the embodiment and the application case, the fault in this case needs to be detected correspondingly rapidly.
There are faults which cannot be detected immediately or for the detection of which complex additional measures are required such as a redundant design of sensor devices, for example. Examples of such faults are firstly breakage of the sensor shaft, i.e. the connection between the drive device and a sensor device, which connection is fixed against rotation, which sensor shaft detects an actual position or an actual rotation speed of the drive device, and faulty operations in the sensor system itself, which bring about apparently correct signals. Such faults cannot be detected in the case of a single-channel sensor device, i.e. in the case of a non-redundant sensor device.
In order to detect such faults, it is naturally possible to provide a second sensor device, i.e. to design the sensor device to be redundant. However, this is firstly associated with corresponding costs, and secondly a corresponding amount of installation space is required for this purpose which is not always available. If, for whatever reasons, a second, redundant sensor device is not provided, it has often simply been assumed in the prior art that, during running operation of the drive device, the state “suspended load” always only occurs for a very short period of time, i.e. the risk of breakage of the sensor shaft and faulty functioning of the sensor device itself during such a standstill time of the drive device can be accepted.